Numerous chemical treatments for cellulosic fibers are known. For example, it is well known that cellulosic fibers may be treated with various alkaline solutions to impart a number of beneficial properties.
Cellulosic fibers may be treated with concentrated caustic solutions to alter the morphology of the fiber structure, in a process referred to as “mercerization.” Mercerization is primarily used to reduce the crystallinity of the cellulose fiber and transforms the ribbon-like cross section of the raw cellulosic fibers into a round shape, providing increased stiffness in the treated fiber. Mercerization processes would also be expected to extract hemicellulose from the fiber. Hemicellulose is generally defined as low molecular weight polysaccharide units inherently present in all cellulosic fibers.
The hemicellulose content of cellulosic fibers may also be reduced by treating the fibers with less concentrated caustic solutions, in an alkaline treatment process commonly referred to as “cold caustic treatment.” Cold caustic treated fibers, particularly cold caustic treated wood pulp fibers, may be used in a variety of applications, including as a raw material in the production of regenerated fibers. Hemicellulose is particularly problematic in the production of regenerated fibers, such as rayon or cellulose acetate, because the hemicellulose clogs spinneret holes during fiber formation.
Cold caustic treatments and mercerization are generally performed by mixing a low consistency cellulose slurry with an alkaline solution, typically a sodium hydroxide solution, and allowing the mixture to steep, or react, for a suitable amount of time. The steeped cellulose fiber is subsequently washed to remove the caustic solution and then may be subjected to downstream processing. The effluent stream produced during washing, aptly referred to as a “hemicaustic” stream, generally contains a mixture of extracted hemicellulose, alkali metal hydroxide and water.
The disposal of hemicaustic streams produced during the alkaline treatment of cellulosic fibers has heretofore been problematic. For example, hemicaustic effluent streams produced by pulping operations have historically been incorporated into alkali streams intended for unrelated processes capable of tolerating the presence of the hemicellulose. However, the number of processes capable of utilizing hemicaustic streams is limited, as the hemicaustic tends to deposit onto pulp fibers when the effluent is used in the conventional cooking, bleaching or caustic extraction processes involved in pulp production. Such deposits render the pulp unfit for many important end uses. Further, extraneous process equipment, e.g. pumps, piping and the like, is required to transport the effluent hemicaustic streams to such unrelated processes. Moreover, a cold caustic process generates more hemicaustic than can be used in the rest of the pulping/bleaching process.
The hemicaustic effluent streams and alkali metal hydroxide solutions associated with alkaline treatment processes also typically require specialized handling procedures to ensure that such materials are not discharged to the environment. Generally, significant hemicaustic effluent streams are produced due to the massive scale involved in cellulose based manufacturing operations, such as pulp mills and the like, further exacerbating the problem. In addition to environmental concerns, the economic value of the hemicellulose contained within the hemicaustic stream, heretofore considered an unwanted by-product, is not recovered within conventional alkaline treatment processes. Thus there remains a need in the art for cellulosic fiber alkaline treatment processes having improved economic and environmental attributes.